1. Field of the Invention
The present invention relates to wireless networking and, more particularly, to wireless networking wherein at least one transceiver is equipped with multiple antennas.
2. Description of the Related Art
In wireless networking devices, especially those designed to support real-time media transport, uninterrupted, high-quality high-speed data transport must be maintained. Wireless data transport systems often must overcome interference, such as multipath (i.e. wherein objects in the environment can reflect a transmitted wireless signal) and spot sources of noise (e.g. wireless phones). Because any interference may have rapidly changing parameters, a system that uses a single transmit antenna and a single receive antenna is limited in its ability to mitigate interference.
To overcome this problem, a system can include multiple receive and/or transmit antennas and use antenna “diversity” to select optimal antennas. Specifically, such a system can select optimal antennas by transmitting/receiving on various combinations of receive/transmit antennas and analyzing the results of such transmitting/receiving.
Wireless data transport systems typically comprise at least two transceivers, wherein each transceiver can include a transmitter and a receiver. When communicating according to, for example, the IEEE 802.11 family of standards, each transceiver may be, in turn, a receiver or a transmitter, but not both simultaneously. When two or more transceivers communicate, they may be considered “communication partners,” assuming alternately the role of transmitter or receiver.
In one embodiment, a receiver may select between two antennas connected to a single receive chain (i.e. receiver components capable of receiving and processing a transmitted signal) based on a received signal characteristic associated with each of the two selectable antennas. Such signal characteristics may include a received signal strength indication (RSSI) and a bit error rate. In one embodiment, the signal characteristics may be determined by processing empirical (generally called “history”) data (e.g. receiving a plurality of transmissions, wherein at least one transmission is received by each antenna, and analyzing the history data in such transmissions)(called “slow diversity” herein). In another embodiment, the signal characteristics may be determined by changing the antenna selection during the reception of a packet (i.e. selecting different antennas during the transmission of a single packet)(called “fast diversity” herein). In yet another embodiment, a hybrid approach may be used. For example, signal characteristics from the second antenna may be gathered briefly during the reception of a packet, and history data for the second antenna created thereby, with reception continuing on the first antenna.
In another embodiment, if a receiver has two antennas, e.g. R1 and R2, and a transmitter has two antennas, e.g. T1 and T2, then the 4 possible antenna combinations include: R1/T1, R2/T2, R1/T2, and R2/T1. Based on any number of performance characteristics, one of these antenna combinations may be better than the others.
In another system, at least two (receive or transmit) antennas operate simultaneously through parallel receive or transmit “chains” (respectively), to perform “beam forming”, wherein antenna signals are combined to maximize performance. An example of beam forming by receiver combining is disclosed in U.S. patent application Ser. No. 10/682,787, entitled “Apparatus and Method of Multiple Antenna Receiver Combining of High Data Rate wideband Packetized Wireless Communication Signals”, filed on Oct. 8, 2003, now U.S. Pat. No. 7,366,089, which issued on Apr. 29, 2008, as well as U.S. patent application Ser. No. 10/682,381, entitled “Apparatus and Method of Multiple Antenna Transmitter Beamforming of High Data Rate . . . Signals”, also filed on Oct. 8, 2003, now U.S. Pat. No. 7,385,914, which issued on Jun. 10, 2008, both of which are incorporated herein by reference. In yet another system, each of the parallel receive or transmit chains may be equipped with a plurality of selectable antennas, thereby permitting a combination of the diversity and beam forming techniques. Note that the terms “multiple receiver combining” (MRC) and “receive beam forming” are used interchangeably herein.
Unfortunately, switching between antenna combinations and/or the analysis of optimal antenna combinations can result in service interruptions. Such interruptions, for a system transporting real-time streaming media, could result in the loss of picture from a television program or the loss of sound from a radio program. Therefore, a need arises for a means of controlling a wireless communication system having multiple receive and/or transmit antennas to avoid interrupting a real-time communication flow.